Centrifugal drives



March 12, 1957 A. c. LANE 2,735,364

CENTRIFUGAL DRIVES Filed Aug. 3, 1955 4 Sheets-Sheet l PL 0V6 FEW SFEEOATTORNEY INVENTOR March 12, 1957 A. c. LANE 2,785,364

CENTRIFUGAL DRIVES 7 Filed Aug. 3, 195a 4 Sheets-Sheet 2 Y //A TINVENTOR jmuk CL/I E M/VE,

ATTORNEY March 12, 1957 A. c. LANE CENTRIFUGAL DRIVES 4 Sheets-Sheet 3Filed Aug. 3, 1953 v R O T N E V m 7 01? CA/I E AKA f(ILLLHHIHHIHHIHHIIIHHIIIHIIIHIHHIHTI ATTORNEY March 12, 1957 A. 0. LANE2,785,364

CENTRIFUGAL DRIVES Filed Aug. 3, 1953 4 Sheets-Sheet 4UHIIHHIHIHIHHHHHIUTITI'P ATTORNEY United States Patent CENTRIFUGALDRIVES Arthur Clive Lane, London, England, assignor to The BritishThomson-Houston Company Limited, a British company Application August 3,1953, Serial No. 372,060 Claims priority, application Great BritainAugust 5, 1952 4 Claims. (Cl. 318197) This invention relates to thecontrol of an alternating current motor of the Schrage type,particularly when used for driving centrifugals such as are used forsugar refining although applicable to other centrifugal devices. Theobject of this invention is to attain as closely as possible to theideal speed-time cycle required for automatic operation with ploughingin the reverse direction for safety purposes and to achieve this type ofcycle with minimum motor losses and without the necessity of employing abrake during normal operation.

This invention consists in the combined use of brush separation andbrush adjustment of a Schrage motor controlled to effect a requiredcycle of operation efliciently. More particularly it consists inproviding a pilot motor to move both brush rockers in oppositedirections and actuators such as thrustors to move either brush rockerindependently.

The invention will be better understood by reference to the accompanyingdrawings, in which:

Fig. 1 schematically illustrates the windings, brushes, brush rockersand slip rings of an alternating current motor of the Schrage type;

Fig. 2A illustrates an ideal speed time cycle curve for a motor of theSchrage type used for driving centrifugal separating devices;

Fig. 2-B illustrates an operating cycle curve such as is obtained byconventional means for speed control of a Schrage type motor;

Fig. 3 is an elevational View, partly in cross section, disclosing mynovel control means for adjusting the brush rockers of a. Schrage typemotor;

Figs. 4, 5 and 6 schematically disclose the brush rockers in differentpositions of adjustment resulting from the actuation of the controlmechanism of Fig. 3, and

Fig. 7 illustrates the speed torque curves of a Schrage type motoremploying the improved control means herein disclosed.

As is known in the art and diagrammatically indicated in Fig. l, aSchrage motor is a polyphase alternating current commutator motor inwhich two sets or groups of movable brushes, i. e. qblX, 2X, oSX, andoil, 2Y and 3Y are provided for speed and power factor control. Thebrush groups are each connected to a separate rotatably mounted brushrocker as schematically represented in Fig. 1. These brush rockers 24and 26 are shown in Fig. 3 and the racks 25 and 27 which cooperate withrockers 24 and 26 are shown in Figs. 4 to 6, inclusive, relative to thestructural elements of the control mechanism.

The rotor 18, best shown in Fig. 3, carries a primary poiyphase winding19. This winding may be a threephase primary winding, for instance asindicated in Fig. l, which is supplied from a power source through sliprings 12. A regulating winding 14 having predetermined points connectedto the segments of the commutator 16 is mounted on the rotor of themachine.

The stator 20 also carries phase windings o1, 2 and 453 which areindicated in Fig. 3 by reference numeral 22 "ice and the ends of each ofthese phase windings are connected to brushes of the two afore-describedsets, 1X, 2X, 3X and oil, 023" and 3Y, respectively, so that the brushesof each corresponding pair are connected across one of the phasewindings of the stator. The distance apart of the brushes of each paircan be varied by speed control will hereinafter be more fully discussed.it" it is assumed that these brushes are the same commutator segment,the stator windings are short-circuited and the motor runs like anordinary induction motor with the difference only that the statorwindings short-circuited instead of the rotor windings as is customary.if the brushes are separated a voltage appears across the statorwindings which is aiding the voltage induced by the rotor windings ifthe separation is in one direction and is opposing if the separation isin the other direction.

The present invention provides a convenient and improved arrangement ofthe brush control means so as to combine the above speed control with anadditional control, which is known as forward or backward shift of thebrush axis, as may be required for acceleration and deceleration periodswith a view to attaining the object first mentioned.

Referring to Fig. 2-13, owing to the inherent characteristics of themotor as determined by its speed, torque and brush positionrelationships, the changes from bottom speed to top speed and vice versatake place by a gradual change of slope as indicated at points A, B, Qand D. This is essentialiy so for two reasons, firstly that the inherentchange of spec with brush position of a Schrage motor obeys the sinelaw, being greatest in the middle of the range and least at top andbottom ends of the range and secondly because of the very great inertiaof the centrifugal load which hinders any sudden change of slope of thespeed-time curve. By conventional brush movement also the motorcharacteristics required for the acceleration, for the retardation andfor he spin and plough periods, cannot be equally realised. Quitedifferent conditions of brush-shift from neutral are required in orderthat these three main operations may be eificiently performed.

Fig. 3 shows a constructional embodiment of the invention, as presentlypreferred. However, it will be understood that the main object of theinvention is to provide a suitable means for the joint operation ofbrush separation and brush shift to produce a required cycle ofoperation with maximum efiiciency. Therefore although the abovedescription has been based on what is considered to be the mostconvenient design it is intended to cover the operation usingalternatives which are mechanically or electrically equivalent, such asactuators other than thrustors for operating the brush shift, orcouplings other than sliding helical pinions enabling both control ofbrush secaration and brush axis shift either separately orsimultaneously as may be required.

The brushgear operated by a pilot motor and thrustors cccrding to thisinvention is shown in Figs. 3, 4, 5 and 6. Pilot motor 23 drives thron hcha'i 2? one brush controlling rocker 24 direct through pinion 22 andrack 25 which is secured to rocker 24. It drives the other brushsupporting rocker 26 through pinion 3t spindle 32 intermeshing helicalpinions 3 and 36, spindle T18, pinion 40 and rack 27. Reference numerals23, and it designate pinions which engage with racks 255 and 27 fixed tothe two brush rockers 24 and 26, respectively. Reference numerals 34 and36 designate helical pinions, 3 being fixed directly on to the spindle32 which carries 33, while 36 is free to slide along the spindle 33which carries pinion 40, but is keyed at i2 to spindle 38 so that 36 and38 must always rotate together. The drive 2328253t* 34 is irreversibleby reason of a reduction gear 44 be- The two thrustors 46 and 48 operateby a link mechanism on to the helical pinion 36, so that this pinion isable to be held in any of three positions as shown in Figs. 4, 5 and 6,depending upon whether both the thrustors are excited, only one thrustorexcited or neither thrustor excited. intermediate positions betweenthose shown in Figs. 4 and 5 or between Figs. 5 and 6 may, of course, beallowed for by simple mechanical means if desired.

With the helical pinion 36 held in the neutral position as in Fig. 5 byreason of having only one thruster excited, i. e. thrustor 43, itfollows that the pitch circle diameters of the pinions 3 5 and 36 mustbe equal to one another and also that the pitch circle diameters of thepinions 23, 3t and 40 must be equal to one another, though notnecessarily equal to that of 3 and 36. This ensures that any movement ofthe pilot motor 23 moves the two brush rockers 24 and 2-6 equally inopposite directions if it is required to maintain a neutral brushsetting at any speed. However pinions 34 and 36 may be of differentpitch circle diameters, and the pitch circles of 23, 3t) and 49 may notbe equal or again the racks 25 and 27 on brush rockers 24 and 26 may notbe equal. Variations in these ratios may be altered at will if, forexample, it is desired to give to the brushgear a progressive anduniform change of shift from neutral in relation to speed. This is veryusual on Schrage motors Where it is customary to set the brushgear moretowards backward shift at low speeds and more towards forward shift athigh speeds, for the modification of inherent neutral characteristics.

With the pilot motor 23 at rest and with the brushgear on neutral as inFig. 5, if now both thrustors 46 and 43 are excited, the helical pinion3% must rise as shown in Fig. 4. Since the train 343il2528-23 is locked,the pinion 34 cannot rotate as as moves axially along spindle 32, sothis axial sliding of 36 must be accompanied by a slight rotation of 3'5relative to 34, i. e. a rotation of spindle 32 and also of pinion 4d anda corresponding movement of the racks 2'7 and rocker 26.

Similarly if both thrustors are switched off they must fall as shown inFig. 6 and the pinion 36 must rotate a small amount in the oppositedirection giving to rack 27 and also to rocker 26 a movement opposite tothat caused by the thrustors rising.

The small movement of rocker 26 in one direction or the other about theneutral setting results in forward or backward shift of the brush axis,and its amount will depend upon the axial travel of 36 relative to 34and to the angle of the helix of the meshing teeth of 34 and 36.

T he pilot motor 23 and the thrustors 46 and 48 can be operatedseparately or together as required by the control scheme desired to beeffected, so that at any time or at any point within the spee range themotor brushgear may be on neutral, with forward shift or with backwardshift.

Thus it can be seen that the action of the thrustors can effect only therack 27 and rocker 26 while the action of pilot motor 23 causes bothrockers to rotate, since irrespective of the axial position of pinion 36in relation to 34, if 34 rotates, 36 must also rotate.

Now referring to Fig. 2-A, the motor is required from B to C and from Dto E to be on neutral brush setting. This is because the load is verylight at these points, and neutral setting gives minimum losses underthese conditions. From A to B the motor brushgear requires to haveconsiderable backward shift in order to give minimum secondary currentand maximum efiiciency under conditions of heavy accelerating (motoring)torque. From C to D, the motor brushgear requires considerable forwardshift in order to give minimum secondary current and maximum efliciencyunder conditions of heavy retarding (generating) torque.

The scheme of operations and the manner in which the 4 desired cycle isobtained will be understood by reference to Fig. 7, which showsspeed-torque curves of a Schrage motor. Speeds in the direction OY areforward speeds and those in direction OY reverse speeds. Torques in thedirection OX are positive or motoring torques and those in direction OXnegative or generating torques.

Curve AA is bottom speed neutral reverse Curve BB is bottom speedneutral forward Curves CC, DD, EE, FF are four only of the infinitenumber of speed-torque curves with brushgear on backward shift, FF beingthat at maximum brush separation for high speed and CC that at maximumbrush separation for bottom speed Curve G0 is the maximum speed onneutral Curves HH, 1], KK, LL are four only of the infinite number ofspeed-torque curves with brushgear on forward shift, HH being that atmaximum brush separation for top speed, and LL that at maximum brushseparation for bottom speed Curve MM is curve for maximum brushseparation bottom speed in reverse with backward shift (AA' is areflection of BB in the XCX axis; MM is a reflection of CC in the XCXaxis) The motor is now assumed to be running at point a on curve AA, i.e., it is running at minimum speed in reverse or neutral, which is thedesired condition for ploughing. When ploughing is finished and the nextcycle of operations is to begin, the motor is plugged (reversal ofprimary switch). Curve BB therefore applies (forward, bottom speed,neutral). Motor is therefore running at b with a large motoring torque.There is then a large accelerating torque trying to reverse the motor uptowards the direction B. At the same time the thrustors are operated toput the brushgear on to backward shift at bottom speed (curve CC), somotor accelerates through zero speed up the line be. The pilot motor isset into operation to move both brush rockers and run up through thespeed range with brushgear still on backward shift along line cdef. Whenbrushgear reaches its maximum speed setting (curve FF) the thrustorsreturn brushgear to neutral (curve GG), the speed maintaining itself atvalue determined by jg. The large accelerating torque has nowdisappeared and motor will run at g for the spinning operation as longas desired. At the end of the spinning period the thrustors operate toput the brushgear on to forward shift (curve HH) so that the motor isnow running at h with a large generating or retarding torque. The pilotmotor now comes into operation to reduce the speed along the line h, j,k, 1, until low speed brush position LL is reached (kinetic energy ofthe centrifugal being transformed into electrical power returned to thesupply during this operation). When point I is reached the motor isplugged to curve MM so that a large motoring torque, represented bypoint In is applied, tending to accelerate the motor in a reversedirection towards M. At the same time the thrustors restore thebrushgear to neutral reverse (curve AA) and motor runs through zero atpoint X up to point a where ploughing can now take place. This completesthe cycle and it can be seen that the whole operation takes place undercomplete control, without the need for any mechanical brake, theessential principle being that reversing, accelerating and retardingtorques are applied precisely when required and of a desirable value, bythe joint operation of brush separation and brush shift.

The operation during the acceleration period known as charging, i. e.filling the basket with massecuite, may take place while the motor isactually undergoing acceleration, or it may be desired to halt theacceleration temporarily at some such speed as 300 R. P. M. so that thecharging takes place at a constant speed. As soon as the charging iscomplete the acceleration is resumed and the cycle completed asdescribed.

Such a halt in the acceleration would be achieved by simultaneouslystopping the pilot motor and putting the brushgear into neutral by meansof the thrustors at the instant that the desired charging speed werereached. The resumption of the cycle would be achieved by restarting thepilot motor and putting the brnshgear into backward shift by thethrustors. In this way a sharp transition between acceleration andsteady speed and between steady speed and further acceleration can beobtained.

The invention is not concerned only with centrifugals for sugar refiningas it can be applied to other centrifugals, nor is it confined to theprecise cycle of operation described. For instance, some sugarcentrifugals do not need to go into reverse for ploughing, and thegeneral scheme applies equally well in this case.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a brush shifting induction dynamoelectric machine of the Schragetype provided with a stator and a rotor, a primary winding on said rotorenergized through slip rings, a commutator connected to a regulatingwinding positioned on the rotor of said machine and having the ends ofeach one of a plurality of independent secondary phase windings on saidstator connected to a pair of brushes which coact with said commutator,a first rotatably mounted brush rocker carrying a first brush groupcomprising one brush of each pair, a second rotatably mounted brushrocker carrying a second brush group comprising the other brush of eachpair, the combination, comprising, an interconnecting drive mechanismfor said first and second brush rockers for angularly adjusting theposition of said brush groups about said commutator, saidinterconnecting drive mechanism comprising two intermeshing helicalgears mounted for relative axial displacement on separate shafts whichare connected to rotate in unison with said first and second brushrockers respectively, a separate drive independently connected to one ofsaid brush rockers by which said one rocker is directly rot-atablydriven and said other rocker is indirectly rotatably driven through saidinterconnecting drive mechanism, and separate power means connected toone of said intermeshing helical gears for axially moving said gearrelative to the other to effect angular displacement of said shafts uponwhich said gears are mounted and of the respective brush groups of saidfirst and second rockers.

2. The dynamoelectric machine as defined in claim 1 wherein the separatedrive comprises an electric motor.

3. A dynamoelectric machine as defined in claim 1 wherein the separatepower means for axially moving said helical gear comprises hydraulicmechanism.

4. A dynamoelectric machine as defined in claim 1 wherein the separatepower means for axially moving said helical gear comprises two thrustorsconnected by linkage means to the axially movable helical gear.

References Cited in the file of this patent UNITED STATES PATENTS1,590,030 Hull June 27, 1926

